Japanese Unexamined Patent Application Publication No. Hei 3-146557 (U.S. Pat. No. 5,229,037) discloses an electrically conductive silicone rubber composition capable of forming an electrically conductive silicone rubber of low hardness and low permanent compression set. This composition comprises a diorganopolysiloxane, spherical silicone rubber particles with an average particle size of 0.1 micrometers to 50 micrometers, a metal based electrically conductive filler, and a curing agent. The spherical silicone rubber particles are typically prepared by curing a silicone rubber composition in an emulsified state in an aqueous solution of a surface active agent and then drying it in a spray dryer, etc. For this reason, the particles contain the above-mentioned surface active agent on their surface or inside. The problem is that the viscosity of the electrically conductive silicone rubber compositions to which they are added increases. Furthermore, attempting to produce electrically conductive silicone rubber with low permanent compression set by adding a large amount of spherical silicone rubber filler causes a marked increase in the viscosity of the resultant electrically conductive silicone rubber composition, which renders preparation of a homogeneous composition impossible.
Japanese Unexamined Patent Application Publication No. 2000-038510 (U.S. Pat. No. 6,017,587) discloses an electrically conductive silicone rubber composition comprising an organopolysiloxane having at least two alkenyl groups per molecule, an organopolysiloxane having at least two silicon-bonded hydrogen atoms per molecule, a platinum group catalyst, a metal based electrically conductive filler, silicone rubber particles, and an organic solvent. The composition is capable of forming highly electrically conductive silicone rubber when a large amount of the electrically conductive filler is used. Although these silicone rubber particles are finely divided particles of irregular shape, the viscosity of the electrically conductive silicone rubber composition obtained by adding them increases. Furthermore, attempting to produce electrically conductive silicone rubber with low permanent compression set by adding a large amount of a spherical silicone rubber filler results in a marked increase in the viscosity of the obtained electrically conductive silicone rubber composition, which also renders preparation of a homogeneous composition impossible.